1. Field of the Invention
The present invention relates to a tunnel valve sensor with a narrow gap flux guide and, more particularly, to such a flux guide which has improved saturation magnetization.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic field signals from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a tunnel valve sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically nonconductive tunneling or barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The tunnel valve sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the tunnel valve sensor for conducting a sense current therethrough. The sense current is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor wherein the sense current is conducted parallel to the major film planes (CIP) of the tunnel valve sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the tunnel valve sensor to the sense current (IS) is at a minimum and when their magnetic moments are antiparallel the resistance of the tunnel valve sensor to the sense current (IS) is at a maximum. Changes in resistance of the tunnel valve sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the sense current (IS) is conducted through the tunnel valve sensor, resistance changes, due to field signals from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the tunnel valve sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel valve sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the tunnel valve sensor at minimum resistance. The dr/R of a tunnel valve sensor can be on the order of 40% as compared to 10% for a spin valve sensor.
The first and second shield layers may engage the bottom and the top respectively of the tunnel valve sensor so that the first and second shield layers serve as leads for conducting the sense current (IS) through the tunnel valve sensor perpendicular to the major planes of the layers of the tunnel valve sensor. The tunnel valve sensor has first and second side surfaces which are normal to the ABS. First and second hard bias layers abut the first and second side surfaces respectively of the tunnel valve sensor for longitudinally biasing the magnetic domains of the free layer. This longitudinal biasing maintains the magnetic moment of the free layer parallel to the ABS when the read head is in the quiescent condition.
Magnetic head assemblies, wherein each magnetic head assembly includes a read head and a write head combination, are constructed in rows and columns on a wafer. After completion at the wafer level, the wafer is diced into rows of magnetic head assemblies and each row is lapped by a grinding process to lap the row to a predetermined air bearing surface (ABS). In a typical tunnel valve read head all of the layers are exposed at the ABS, namely first edges of each of the first shield layer, the seed layer, the free layer, the barrier layer, the pinned layer, the pinning layer and the second shield layer. Opposite edges of these layers are recessed in the head. The barrier layer is a very thin layer, on the order of 20 xc3x85, which places the free and pinned layers very close to one another at the ABS. When a row of magnetic head assemblies is lapped there is a high risk of magnetic material from the free and pinned layers being smeared across the ABS to cause a short therebetween. Accordingly, there is a strong-felt need to construct magnetic head assemblies with tunnel valve heads without the risk of shorting between the free and pinned layers at the ABS due to lapping.
A scheme for preventing shorts across the barrier layer of the tunnel valve sensor is to recess the tunnel valve sensor within the head and provide a flux guide between the ABS and the sensor for guiding flux signals from the rotating magnetic disk to the sensor. Typically, the ferromagnetic material of the flux guide is required to be stabilized by hard bias layers on each side of the flux guide. The prior art ferromagnetic material employed for the flux guide is nickel iron (NiFe). Generally, the thickness of a nickel iron flux guide layer is 100 xc3x85 at the ABS in order to provide sufficient magnetization for detecting the field signals from the rotating magnetic disk. There is a strong-felt need to reduce this thickness in order to increase the linear read bit density of the read head. When the linear read bit density is increased more magnetic bits can be placed per linear inch along a track of the rotating magnetic disk which increases the storage capacity of the computer.
In the present invention the flux guide of the tunnel valve sensor includes an iron nitride (FeN) layer which has twice the magnetization of nickel iron (NiFe). In one embodiment of the invention the flux guide is a single layer of iron nitride with a thickness one-half of the prior art nickel iron layer. Accordingly, the single iron nitride flux guide layer can be 50 xc3x85 instead of 100 xc3x85 which reduces the read gap of the read head by 50 xc3x85. As discussed hereinabove, this increases the linear read bit density of the read head. In a preferred embodiment, however, a second layer of nickel iron molybdenum (NiFeMo) is employed for decreasing the uniaxial anisotropy (HK) Of the flux guide. When the uniaxial anisotropy (HK) is reduced the magnetic moment of the flux guide is more responsive to field signals from the rotating magnetic disk which increases the sensitivity of the read head. An exemplary second embodiment employs 37 xc3x85 of iron nitride and 25 xc3x85 of nickel iron molybdenum which has a magnetization equivalent to 100 xc3x85 of nickel iron. In this example the iron nitride layer increases the magnetization while the nickel iron molybdenum layer increases the uniaxial anisotropy (HK). Both of these layers increase the permeability of the flux guide as compared to the permeability of a nickel iron flux guide. In still a further embodiment of the present invention a plurality of iron nitride and nickel iron molybdenum layers are laminated with sufficient thicknesses to provide magnetization equivalent to a 100 xc3x85 thick nickel iron flux guide layer which provides the aforementioned reduced read gap and reduced uniaxial anisotropy (HK).
An object of the present invention is to provide a tunnel valve sensor with a flux guide which has a more narrow read gap.
Another object is to provide a tunnel valve sensor with a flux guide which has improved magnetization.
A further object is to provide a tunnel valve sensor with a flux guide which has improved magnetization and improved uniaxial anisotropy (HK).
Still another object is to provide methods for making the aforementioned flux guides for the tunnel valve sensor.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.